Bihlio graphical Notices. 263 



ideas of the nature of the facts indicated by these terms." He even 

 defines a " genus," and illustrates the definition by reference to the 

 characteristics of certain genera of Lamellibranchs, showing their 

 mutual relations. We do not find, however, that he has ventured 

 on the definition of " species." The fact is that, when once the 

 doctrine of evolution is ad nitted in all its entirety, the ideas 

 attaching to such expressions as " genus " and " species " lose some- 

 what of their definition. Nevertheless they are necessities in 

 classification ; and even if they do not absolutely exist in nature, it 

 becomes necessary, as by a sort of legal fiction, to presume that they 

 do exist, for purposes of scientific arrangement. But the palaeontolo- 

 gist lias to deal with the element of time in addition to his other 

 difficulties, and thus for him, far more than for the simple student 

 of recent life, does it become necessary not to place too rigid a 

 definition on "genus" and "species." There are occasions when 

 we must dare to be illogical. 



After describing some of the varieties of deposit, Prof. Seeley 

 discusses the phenomena attendant upon Life, such as its succession 

 in time, migration, the origin of faunas, extinction of species, 

 homotaxis, &c, concluding with an account of the existing distribu- 

 tion of life, and the relations of living to fossil forms. Of the col- 

 lateral subjects in this connexion he alludes to the climatal conditions 

 of ancient seas, which he considers must rest on physical evidence. 

 " Ice-scratched stones, glaciated rocks, aud boulder-clay may prove 

 conditions of great cold ; but we are acquainted with no physical 

 evidence that would demonstrate heat as a climatic condition of the 

 earth." This may be so, but surely there exists biological evidence 

 of temperature in the presence of reef-building corals, which now 

 require an isochryme not lower than 69° F. Such an inference has 

 been held legitimate by Prof. Dana, in common with most geolo- 

 gists ; and it is certainly a singular coincidence that a life assem- 

 blage presenting some analogies with that of the Jurassic should, 

 at the present day, be found in and about Australia, where reef- 

 builders abound. Prof. Seeley is a bit of an iconoclast, and few 

 things give him more pleasure than to upset, or try to upset, a pre- 

 valent belief. We say nothing about such a case as that of the 

 Stonesfield mammals, for it is in dealing with the Mammalia that 

 the most erroneous inferences as to climate have been drawn, as is 

 very aptly pointed out by the author in the case of the mammalian 

 remains of our valley-gravels. 



Finally, we are presented with a brief abstract of the succession 

 of life on the earth in geological time — not the least useful portion 

 of the entire volume, and certainly the most suitable for a manual, 

 since there is a great amount of condensed information well brought 

 up to date, and less of the theoretical than elsewhere. The figures 

 too are instructive and germane to the subject. It is almost un- 

 necessary to add that the portion devoted to the Yertebrata is par- 

 ticularly good. 



Many of the views which have been developed and perfected in 

 the present work were originally brought forward by the author in 



